The present invention is generally related to transfer cases for use in four-wheel drive vehicles and, more particularly, to an improved two-speed on-demand transfer case equipped with a ball-ramp clutch assembly.
In view of increased consumer popularity in four-wheel drive vehicles, a plethora of power transfer systems are currently being utilized in vehicular driveline applications for selectively directing power (i.e., drive torque) from the powertrain to all four wheels of the vehicle. In many power transfer systems, a transfer case is incorporated into the driveline and is operable in a four-wheel drive mode for delivering drive torque from the powertrain to both the front and rear wheels. Many conventional transfer cases are equipped with a mode shift mechanism that can be selectively actuated to shift between a two-wheel drive mode and a four-wheel drive mode. In addition, many transfer cases also include a range shift mechanism which can be selectively actuated by the vehicle operator for shifting between four-wheel high-range and low-range drive modes.
It is also known to use xe2x80x9con-demandxe2x80x9d power transfer systems for automatically biasing power between the front and rear wheels, without any input or action on the part of the vehicle operator, when traction is lost at either the front or rear wheels. Modernly, it is known to incorporate the xe2x80x9con-demandxe2x80x9d feature into a transfer case by replacing the mechanically-actuated mode shift mechanism with a clutch assembly that is interactively associated with an electronic control system and a sensor arrangement. During normal road conditions, the clutch assembly is typically maintained in a non-actuated condition such that drive torque is only delivered to the rear wheels. However, when the sensors detect a low traction condition, the clutch assembly is automatically actuated to deliver torque xe2x80x9con-demandxe2x80x9d to the front wheels. Moreover, the amount of drive torque transferred through the clutch assembly to the non-slipping wheels can be varied as a function of specific vehicle dynamics, as detected by the sensor arrangement. This on-demand clutch control system can also be used in full-time transfer cases to automatically bias the torque ratio across an interaxle differential.
In some two-speed on-demand transfer cases the range shift mechanism and the clutch assembly are independently controlled by separate power-operated actuators. For example, U.S. Pat. No. 5,407,024 discloses a two-speed range shift mechanism actuated by an electric motor and a clutch assembly actuated by an electromagnetic coil. In an effort to reduce cost and complexity, some transfer cases are equipped with a single power-operated actuator that is operable to coordinate actuation of both the range shift mechanism and the clutch assembly. In particular, U.S. Pat. Nos. 5,363,938 and 5,655,986 each illustrate a transfer case equipped with a motor-driven sector with cam surfaces adapted to coordinate actuation of the range shift mechanism and the clutch assembly for establishing a plurality of distinct two-wheel and four-wheel drive modes. While transfer cases equipped with such coordinated actuation systems have been commercially successful, a need exists to develop alternative systems which further reduce the lost and complexity of two-speed on-demand transfer cases.
Accordingly, it is an object of the present invention to provide a transfer case equipped with a two-speed range unit, a clutch assembly, and a power-operated actuation mechanism for controlling coordinated actuation of the range unit and the clutch assembly.
It is another object of this invention that the transfer case be interactively associated with a control system for controlling operation of the power-operated actuation mechanism to establish various four-wheel high-range and low-range drive modes.
It is further object of the present invention to locate the clutch assembly across an interaxle differential to provide automatic torque biasing in a full-time four-wheel drive mode.
As a related object, the clutch assembly can be operably disposed between the front and rear output shafts to provide automatic torque transfer in an on-demand four-wheel drive mode.
Another object is to provide a synchronized range unit for permitting on-the-move shifting between high-range and low-range drive modes.
According to a preferred embodiment, a transfer case is provided with a range unit, an interaxle differential, a clutch assembly, a power-operated actuation mechanism, and a control system. The range unit includes a planetary gearset driven by an input shaft, and a synchronized dog clutch assembly for releasably coupling one of the input shaft or an output component of the planetary gearset to an input member of the interaxle differential. The interaxle differential further includes a first output member driving a first output shaft, a second output member operably driving a second output shaft, and a gearset transferring drive torque from the input member to the first and second output members. The clutch assembly is a multi-plate friction clutch operably disposed between the first and second output shafts. The power-operated actuation mechanism includes an electric motor assembly, a drive shaft driven by the motor assembly, a range actuator assembly and a clutch actuator assembly. The range actuator assembly includes a cam rotatively driven by the drive shaft, and a shift fork having a follower segment retained in a groove formed in the cam and a fork segment retained in a groove formed in a shift collar associated with the synchronzied dog clutch assembly. Rotation of the cam results in movement of the shift collar between high-range (H), neutral (N) and low-range (L) positions. The clutch actuator assembly includes a ball-ramp unit and a gear assembly. The ball-ramp unit includes a first plate, a second plate, and balls retained in ramped grooves formed between the first and second plates. The gear assembly includes a first gear driven by the drive shaft, a second gear fixed to the first plate of the ball-ramp unit, and a third gear meshed with the first and second gears. The control system is adapted to control the magnitude and direction of rotary motion of the drive shaft through controlled energization of the motor assembly.
The power-operated actuation system of the present invention is arranged to permit sufficient bi-directional rotation of the drive shaft to move the shift collar between its H and L positions without causing the ball-ramp unit to engage the multi-plate clutch. However, once the shift collar is positively located in either of the H or L positions, continued rotation of the drive shaft causes actuation of the ball-ramp unit for exerting a clutch engagement force on the multi-plate friction clutch.